1. Field of the Invention
This invention relates to an electrophotographic image formation method for use in the fields of electrophotography, electrostatic recording and electrostatic printing, and more particularly, to a multi-color electrophotographic image formation method of a digital type, capable of producing high quality multi-color or full-color images.
2. Discussion of Background
Although various electrophotographic image formation methods are disclosed, for example, in U.S. Pat. No. 2,227,691, and Japanese Patent Publications 42-23910 and 43-24748, the most common method is such that a latent electrostatic image is first formed on the surface of a photoconductive member and then developed with a developer (toner) to obtain a visible toner image, or when necessary, the thus obtained toner image is transferred to a sheet of paper or the like and then fixed thereon by application of heat or pressure, or both, or by the use of a vaporized solvent to obtain visible images.
A multi-color electrophotographic image formation method of an analogue type is disclosed, for example, in U.S. Pat. No. 2,962,374. In this method, the colors of an original image are separated and converted into electric signals corresponding to at least three colors of yellow, magenta and cyan, and the surface of a photoconductor is electrically charged in accordance with each of the color signals to form a latent electrostatic image corresponding to each of the separated colors, and each of the latent images is then successively developed with a corresponding color developer of yellow, magenta or cyan to obtain multi-color images.
In addition, various proposals are now being made on a multi-color electrophotographic image formation method of a digital type. In this method, an original image is subjected to color separation to obtain color signals, which are then subjected to an A/D conversion to obtain digital color signals. By conducting a predetermined computation, recording signals are obtained from the digital color signals, and latent electrostatic images are formed on the surface of a photoconductor in accordance with the digital signals. The latent electrostatic images are then developed with color developers to obtain visible multi-color images.
As described above, in the digital multi-color electrophotographic image formation method, the optical information is subjected to computation processing, so that a masking treatment and an undercolor removal (UCR) treatment can be carried out, which cannot be successfully carried out in the conventional multi-color electrophotographic image formation method of an analogue type.
In the case where the UCR treatment is conducted, a latent image corresponding to black color can be developed with a black developer to obtain a black image, instead of successively overlapping yellow, magenta and cyan toner images. The gray balance of the obtained images can thus be improved; in other words, excellent half-tone images are obtainable. In addition, the thickness of the toner layer of the black area obtained by using a black developer is thinner than that of the black area obtained by superimposing yellow, magenta and cyan images. The digital method is therefore economically advantageous, and can provide images without the problem of the curling of a transfer sheet.
A black developer for use in the multi-color electrophotographic image formation method, which is hereinafter referred to as a black developer for process color, is required to assume a black color just like the conventional black developer, which is widely used for a copying apparatus for producing black and white images, as a matter of course. In addition, the spectral reflectance characteristics of the black developer for process color is required to show the panchromatic absorption in the visible light range. According to the multi-color electrophotographic image formation method of a digital type, a black developer for process color is used to form a pictorial images as well as a character image by the UCR treatment. Therefore, the optical properties of the black developer for process color have significant effects on the image quality of the obtained images.
With the above-mentioned importance of the black developer for process color in the electrophotographic image formation method of a digital type taken into consideration, it is essential for the black developer to be provided with excellent color mixing properties, high transparency, adequate coloring power with sufficient gradation, and good image fixing properties.
The conventional black developers used in the copying apparatus for producing black and white images are designed for the purpose of reproducing mainly character images. Therefore, the above black developers are desired to produce images on a transfer sheet with a sufficient density only by a single layer of toner particles. Thus, the optical properties of such black developers, for example, the transparency, capabilities of exhibiting bright mixed-color and a controllability of a coloring power are almost ignored.
When the conventional black developers are used as black developers for a process color, not only the gradation of images becomes poor, but also color reproduction is adversely affected because the optical properties of the conventional black developers do not meet the requirements for the multi-color electrophotographic image formation.
Furthermore, the surface roughness of the obtained images, characteristic of the multi-color electrophotographic image formation method of a digital type, is unfavorably emphasized when used in the digital-type multi-color electrophotography. The reason for this is that the optical properties of the conventional black developers, such as transparency and the coloring power thereof are not considered. The conventional black developers are designed only in due consideration of the reflection density.